Dynamic light scattering of light absorbing model solutions

Date of Completion

January 2007


Chemistry, Analytical




Dynamic light scattering (DLS) is a powerful tool for the characterization of polymers, biomolecules and other macromolecules. It is a rapid, non-invasive technique which can be applied to in situ analyses. However, the difficulty and the complexity of acquisition and interpretation of light scattering data from polymeric and colloidal solutions that absorb light typically restrict the application of DLS to transparent colorless solutions. This restriction excludes many important classes of analytes e.g. light absorbing proteins, conducting polymers and nanotubes. The autocorrelation functions obtained from the DLS of light absorbing solutions show oscillation which is not observed in homodyne DLS of non-light absorbing samples. The oscillation was attributed to the absorbed light which is dissipated as heat causing a cascade of heating effects including thermal gradient, thermal blooming, density and viscosity decrease, refractive index change and convection. ^ The phenomenon was studied using model solutions consisting of non-light absorbing scatterers where the optical density was independently changed by the incorporation of a light absorbing dye. The oscillatory autocorrelation functions previously observed in light absorbing mixtures e.g. polyaniline, cytochrome c/cytochrome c peroxidase, and gold clusters are reproduced in the light absorbing model solutions containing bimodal-sized scatterers. ^ No oscillation was observed from light absorbing solutions of monodispersed scatterers. The model function for the scattering from a mixture with bimodal-sized distribution in a convective flow field developed by Sehgal and Seery fit the oscillatory autocorrelation functions obtained for the light absorbing model solutions. The degree of temperature rise and coning as a function of laser power and dye concentration was studied. The influence of laser power and optical density, scatterer concentration, temperature and polydispersity on the oscillatory correlation functions was evaluated. The model function was tested and applied to single-walled carbon nanotubes to determine its diffusion coefficient. ^